Seismic risk assessment for oil and gas pipelines - Seismic Risk Analysis and Management of Civil Infrastructure Systems - page 689

less than about 1.5 m/s and the lower bound value for the inverse of the
slowness is 2000 m/s, and an amplifi cation factor of 1.5 for nonlinear effects,
an upper limit on the strain that can be generated from wave propagation
is 0.056%. With the exception of early oxyacetylene welded pipelines
and some pre-1945 pipelines fabricated with early arc welding processes
(Honegger, 1999), oil and gas pipelines should be capable of tolerating
longitudinal strains of 0.5%, the nominal yield strain for API 5L pipe.
Assuming a longitudinal strain of 0.18% from internal pressure, the remain-
ing longitudinal strain capacity available for oil and gas pipelines to accom-
modate wave propagation is nearly six times greater than what is expected
to be typically generated from earthquake shaking. For this reason, wave
propagation is an insignifi cant hazard to modern welded steel oil and gas
pipelines and only need be considered for older pipelines when the axial
strain capacity of the older pipelines is less than about 0.25%.
25.6.2 Site-specifi c estimation of permanent
ground deformations
In general, for a given site, the site-specifi c ground displacements are esti-
mated based on the following key steps: (a) geotechnical investigation to
understand the site-specifi c soil and groundwater conditions; (b) assessment
of site-specifi c loading parameters (e.g., changes to groundwater level under
different weather conditions, and ground shaking intensity due to an earth-
quake) and ground response for the identifi ed loading levels; (c) assessment
of the geotechnical stability of the site; and (d) assessment of ground dis-
placement hazard using empirical and/or mechanistic approaches (e.g.,
seismic slope stability, liquefaction-induced ground movements, and post-
earthquake bearing capacity).
The deformation modulus and shear strength of liquefi ed soil are the key
factors determining the extent of ground displacements. These parameters
are often necessary for the assessment of seismic vulnerability and design
of mitigation measures. Numerical approaches based on soil mechanics
principles provide a means to estimate displacement patterns, but they
often suffer from a lack of rigorous calibration with actual earthquake data.
25.6.3 Regionalassessmentsofpermanent
ground deformations
When it is necessary to assess the seismic vulnerability of a wide-area pipe-
line networks, regional approaches are required to determine the ground
displacement hazards. The approach proposed by Youd and Perkins (1987)
provides a general method for mapping the liquefaction susceptibility based